1. Field of the Invention (Technical Field)
The invention generally relates to dielectric thin-films belonging to the Ba.sub.x Sr.sub.y Ca.sub.1-x-y TiO.sub.3 class of perovskite oxide and fabrication methods thereof.
2. Background Art
Thin-film ferroelectric materials have application for various electronic devices, such as dynamic and ferroelectric random access memories ("DRAM" and "FRAM"). Thin-film ferroelectric materials are also widely used in the development of new microwave devices such as frequency agile filters, phase shifters, and tunable high-Q resonators as taught generally in L. A. Knauss, et. al., Appl Phys. Lett. 69:25-27 (1996) and P. Bhaftacharya, et. al., Jpn. J. Appl Phys., 32:4103-4106 (1993).
The dielectric currently utilized in DRAMS and for microwave applications is SiO.sub.2 or a silicon oxide/nitride composite layer ("ONO") with a relative dielectric constant of 6. However, as integrated circuit devices move toward higher and higher integration densities, severe demands are placed on the device design, particularly with respect to squeezing storing capacity into a smaller cell space. A capacitance of about 9 fF/.mu.M.sup.2 appears to be the maximum achievable value for ONO type of materials. As a result, since the mid-1980's, there has been an increasing effort to replace the ONO dielectric with an alternative dielectric having a substantially higher capacitance per unit area.
Most attention has been focused on (Ba.sub.x Sr.sub.1-x)TiO.sub.3 ("BST"), as these materials possess high dielectric constants (.epsilon..sub.r) and low loss (tan .delta.). At room temperature, single crystal SrTiO.sub.3 has a very low loss (tan .delta.&lt;10.sup.-4) but also a low dielectric constant. On the other hand, BaTiO.sub.3 has very high dielectric constants but high loss. Mixing Sr and Ba has resulted in BST materials with high dielectric constants and improved tan .delta. over BaTiO.sub.3. BST is a ferroelectric with the perovskite structure. The BST solid solution also shifts the Curie point of BaTiO.sub.3 at 130.degree. C. to around room temperature for Ba.sub.0.7 Sr.sub.0.3 TiO.sub.3, thus achieving the maximum permittivity around the operating temperature.
A method of making various elemental compositions of BST type materials is taught by Azuma, et.al., in U.S. Pat. No. 5,723,361. Azuma uses molecular precursors, preferably metal carboxylates or metal alkoxides dissolved in an organic solvent such as xylene. The thoroughly mixed solution is then coated on a substrate by a "spin-on" deposition process. Following each spin coat the solvent is removed by a low temperature drying process. The desired thickness of the final film thus depends upon the number of spin-dry cycles in the process. Although Azuma suggests the combination of the metal calcium with the core BST material the method of Alzuma requires the spin coating of a liquid metal precursor, preferably in the form of a metal carboxylate or alkoxide. Also, the traditional co-deposition method of Alzuma permits only one specific material composition to be made per deposition process. That is, the final stoichiometry of the BST type material is uniform throughout the deposited material, and is predetermined by the solution of mixed metal concentrations. In the present invention, the unique deposition process results in a BST type material that is non-uniform throughout the deposited material, and thus allows the testing of various metal compositions from a single deposition process.